Occasionally, the extra chromosome doesn't appear until early in the development of the embryo.

CNN: Triple X syndrome

But are these genes unique to the fly, or do homologous genes determine the development of a human embryo?

ECONOMIST: Genetic science

Like ES cells, maGSCs can spontaneously differentiate into the three basic tissue layers of the embryo - and contribute to the development of multiple organs when injected into embryos.

BBC: NEWS | Health | Testicle cells may aid research

That furnishes the egg with enough water to carry out the reactions of embryo development.

ECONOMIST: Fish genetics

The theory is that the protein in sperm which triggers chemical changes in the egg, leading to embryo development, is defective or missing from the sperm of some infertile men.

BBC: Sea squirts aid fertility research

By studying the expression patterns of individual genes, cloners have found that certain genes which should be turned on in early development, such as those that control the implantation of the embryo into the uterus, are activated much later in cloned embryos.

ECONOMIST: Cloning around